The invention relates to the field of automated mapping systems and more particularly to a system for automatically acquiring planimetric and topographic data of a celestial body in near real time.
Many techniques have been devised for mapping the surface of a celestial body, such as the earth, to provide both planimetric (two-dimensional) and topographic (elevation) information. Prior to the advent of orbiting space satellites, most such data acquisition and mapping was done by aircraft which carried photographic cameras suitable for obtaining stereoscopic coverages. Such techniques are still useful for providing high resolution stereoscopic views of relatively small areas of the surface, but such techniques are limited in that they involve an enormous amount of tme and funds to map large areas of the earth. In addition, in order to produce maps containing elevational information from the stereo photographs, each such stereo pair must be precisely oriented and calibrated using complex normally hand-driven stereoplotter apparatus. The resultant map is at least in part, commonly drawn by hand with the topographic contours being added by compilation from the model created by the stereo photographs.
Satellite imaging systems have been used for the past ten years for recording in an orthographic (two-dimensional) mode surface features of the earth. Such satellites as Landsats 1, 2, and 3 are characterized in having near polar, sun-synchronous circular orbits which allow continuous mapping of the entire earth's surface (excluding a small portion of the polar and cloud covered areas) in approximately eighteen days. Landsats and their Apollo predecessors have successfully used conventional photographic, Vidicon, and mechanical scanner-type imaging systems to record planimetric and radiometric data of the imaged area below the path of the moving satellite. However, none of the currently available earth imaging satellites has the ability to properly resolve the third dimension in a continuous mode suitable for automated processing.
With conventional photographic systems, a great deal of the total cost and time arises in the image data processing. Even with Landsat, which is a digital system, data processing is the principal item of concern. Currently, U.S. based processing facilities are limited to processing approximately 100 Landsat orthographic images (185 km by 160 km; 79 m by 79 m pixel) per day. It can thus be appreciated that stereoscopic imaging of the earth, using conventional imaging wherein two distance separated images are made would impose an even greater burden on present processing facilities since not only would each image have to be geometrically and radiometrically corrected, but each stereoscopic pair of images would have to be correlated and digitized point by point to derive topographic data therefrom.
In order to overcome some of the difficulties of stereographic mapping techniques, it has been proposed (Katz, Height Measurements with the Stereoscopic Continuous Strip Camera, Photogrammetric Engineering, Vol. 18, No. 1 Mar. 1952, pp. 53-62) that height measurements could be made with an airborne stereoscopic continuous-strip type camera. It has also recently been proposed that conventional stereo image pairs can be scanned using epipolar-scan principles, thus reducing the image correlation (for elevational information) from a two- to a one-dimensional task. (Helava and Chapelle, Epipolar Scan Correction, Bendix Technical Journal, Spring, 1972, pp. 19-23). Epipolar scanning refers to a pair of scan lines in a plane which contains the air base of a stereo pair of images. It may also be applied to scan lines in a plane involving two satellite positions from which the earth is viewed and is referred to herein as the epipolar plane concept.
Apparatus has also been developed for automatically scanning stereo photo pairs using epipolar scanning and one-dimensional digital image correlation (Scarano and Brumm, A Digital Elevation Data Collecting System, Photogrammetric Engineering and Remote Sensing, vol. 42, No. 4, Apr. 1976, pp. 489-496). Thus, the concept of reducing photogrammetric data fro two- to one-dimension is well established, but the prior art does not describe the possibility of imaging the earth directly in stereoscopic digital form suitable for one-dimensional processing utilizing the epipolar plane concept.
Recent developments in solid state imaging detector technology have produced linear solid state photodetector array sets capable of imaging upwards of 20,000 separate picture elements (pixels) along the face of the array. Such arrays have desirable properties such as low power use, multi-spectral capabilities, low weight, and no moving parts which is a definite advantage in the satellite environment where rotational stability of this type of mapping satellite considered should be in the order of 10.sup.-6 degrees per second for optimum results.
Thus, there is a need for a automated satellite mapping system which has high stability, a minimum of moving parts, and the ability to image the earth in both planimetric and topographic modes and in one or a variety of spectral bands. Such a satellite mapping system must also utilize the epipolar plan concept to reduce data correlation for elevational information to one-dimensional processing in near real time.
One objective is to provide a method and apparatus for imaging the surface of a body to provide high resolution, panchromatic or multispectral data of the body.
Another objective is to provide a method and apparatus for imaging the surface of a body wherein both planimetric and topographic data are acquired.
The final objective of the invention is to provide a method and apparatus by which the planimetric and topographic data can be processed in near real time to produce visual images and a wide variety of planimetric, topographic and thematic map products.